Showing papers by "Eberhard Lehmann published in 1999"

Neutron radiography of irradiated fuel rod segments at the SINQ: loading, transfer and irradiation concept

Abstract: The thermal neutron source SINQ will be used for the examination of irradiated nuclear fuel segments and highly activated samples. The corresponding installation, the transfer cask, which houses the samples, and the loading and handling procedures are described. First experiments to investigate the hydride distribution in fuel claddings are reported.

Neutron signal transfer analysis

Abstract: A new method called neutron signal transfer analysis has been developed for quantitative determination of hydrogenous distributions from neutron radiographic measurements. The technique is based on a model which describes the detector signal obtained in the measurement as a result of the action of three different mechanisms expressed by signal transfer functions. The explicit forms of the signal transfer functions are determined by Monte Carlo computer simulations and contain only the distribution as a variable. Therefore an unknown distribution can be determined from the detector signal by recursive iteration. This technique provides a simple and efficient tool for analysis of this type while also taking into account complex effects due to the energy dependency of neutron interaction and single and multiple scattering. Therefore this method provides an efficient tool for precise quantitative analysis using neutron radiography, as for example quantitative determination of moisture distributions in porous building materials. A comparison between neutron signal transfer analysis and conventional methods has been made by simulating an experiment with a sample containing a known moisture distribution by Monte Carlo calculation. By using the neutron signal transfer method the error is less than 3%. Whereas the calibration curve method overestimates the moisture content by a factor in the range around 10% and the exponential attenuation method underestimates the moisture content by a factor in the range of 2.

Application of new radiation detection techniques at the Paul Scherrer Institut, especially at the spallation neutron source

Abstract: The demands on modern irradiation detection systems are diverse, encompassing spatial resolution, dynamic range, sensitivity and reproducibility. Nevertheless, there are two important new methods which can satisfy most of these demands in several applications: camera based systems and imaging plates. Imaging plates have primarily been used as γ- and β-sensitive detectors in biology and medicine, but are now available also as neutron sensitive systems. These methods are ideally suited for applications in neutron radiography because of their high sensitivity, linearity and digital output. Image processing, quantification of the image data and automated pattern recognition can easily be performed using modern software tools. The imaging plate system at PSI is shared between groups in reactor physics, radiation protection, biology, proton therapy and nuclear medicine. The collected experience from these different interests establishes the basis for a most effective application of this technique. The utilisation of detector systems based on CCD-cameras have other advantages, like low acquisition time, high frame rates, reproducibility as well as high dynamic range. These detectors are applied to investigations of time dependent processes, distribution analyses and quantitative studies. The new radiography facility NEUTRA at the spallation source SINQ provides excellent conditions for neutron radiography studies. Some first examples of investigations with imaging plates and the CCD-camera system are given.

Applications and characteristics of imaging plates as detector in neutron radiography at SINQ

Abstract: Imaging plate technique is a commonly accepted method in many fields as in medicine, biology and physics for detection of the distribution of β- and γ-radiation or X-rays on large areas. Recently a new type of imaging plate sensitive to neutrons has been developed. The storage layer is doped with gadolinium, which, after absorption of neutrons, produces radiation detectable by the same sensitive crystals used in conventional imaging plates. At the spallation neutron source, SINQ, at the Paul Scherrer Institut (CH) some of the characteristics of the neutron radiography station in combination with the imaging plate technique were investigated. The intensity distribution of the source was measured to check the accuracy for quantification of the image data. Also, the reproducibility of results obtained by this detection system was stated. For a test object, the high selectivity for different neutron absorption is demonstrated at details with low contrast. The obtainable spatial resolution was determined recording an image of a gadolinium edge, from which the line spread function and the modulation transfer function can be calculated. By use of conventional X-ray sensitive imaging plates the exposure time for a indirect detection method using a dysprosium foil can be reduced significantly. For the measurement of the latent image in the neutron activated foil it is sufficient to excite the imaging plate by the radiation of the isotope 165 m Dy with a short life time. The possibility to measure effective cross-sections of materials with pretty strong absorption by imaging plates as detector because of their high dynamic range was proved as well.